Authorities released this sketch of the suspect in the Aug. 6 rape of a 21-year-old woman.

It’s happened to lots of us. Rush hour arrives, and the flight departs soon, and there’s no cab to be found. Or the bars have just closed, and a half-dozen taxis, their seats already occupied, zip past.

Then, a shiny black sedan pulls up, offering a ride. Or a not-so-shiny car with a magnetic logo stops.

What do you do?

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That question is being asked with new urgency after alleged attacks on two women who climbed into unlicensed vehicles within the past two weeks.

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Police are giving straightforward tips on how to distinguish licensed cabs from the unlicensed vehicles that ply the road. But it can be harder to figure out whether one of those black Town Cars is safe to use.

Here are the basics: When looking for a licensed taxi, check for a medallion, the swatch of metal on a vehicle that shows it is registered with the Boston Police Department or another city. The logo and other lettering on the cab should be permanent and made of vinyl rather than a temporary magnet slapped onto the side. Boston taxis are primarily white in color, though that’s different in Brookline and Cambridge. And drivers should have a posted photo identification.

“Whether you’re hailing a cab or being picked up at a cab stand, it’s got to be a real taxi cab, licensed with a city,” said Alfred LaGasse, chief executive officer of the Taxicab, Limousine, and Paratransit Association, a national organization for taxi operators. “You’ve got to know that they’ve been checked out by your local police department.”

Mike Fogarty, president of the Americas at Tristar Worldwide Chauffeur Services, which operates in Boston, New York, London, and Hong Kong, said people hailing a taxi from the streets should seek only cars with medallions and proper licenses.

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That means, beware of black Lincoln Town Cars that magically appear, unbidden. An unmarked livery vehicle has no business picking up passengers without a prearranged appointment and pick-up time.

If you’re unsure about a vehicle, ask for its Massachusetts Port Authority license — a reputable chauffeur company will have a license with the airport that verifies its drivers have undergone criminal background checks and proper training.

Fogarty urges riders to be wary if an unmarked Town Car approaches on the street. “You’re looking for a problem,” he said. “What recourse do you have? Who do you call if something goes wrong? You need to use a company that’s properly licensed.”

That was not the case for two women involved in separate recent incidents.

And at 2 a.m. Sunday, another woman was sexually assaulted after she tried to hail a taxi in Allston and was instead picked up by a man driving a dark, four-door, midsized sedan who said he would take her to Cambridge.

‘Whether you’re hailing a cab or being picked up at a cab stand, it’s got to be a real taxi cab, licensed with a city.’

The attacks have led to calls for increased regulation in the industry and a crackdown on imposter taxis that solicit customers without an official hackney unit medallion.

Additionally, concerns have been raised about smartphone applications that allow prospective customers to send out a signal to nearby cabs that they need a ride. If a taxi accepts the request, the driver heads directly to where the passenger is waiting. But some of these apps also allow chauffeur companies or people using personal cars to respond.

Meghan Joyce, general manager of the smartphone app company Uber Boston, uses both licensed taxis as well as Town Cars and people using personal cars who pass criminal background checks.

“It’s terrible to think of something like this happening in the city of Boston,” Joyce said, referring to the attacks. “But technology is helping us ensure that our transportation is as safe as possible.”

Hailo’s app employs only taxis licensed by the city’s hackney unit. Additionally, just like Uber, the app allows people to see in advance the name and photo of the person designated to pick them up, so there is little guesswork. “You get information that sort of adds an element of comfort and security to your experience, so when the cab arrives, you know what to look for,” Kafka said.

More than convenience, Kafka said, the benefit of e-hailing smartphone apps is the safety that it ensures.

“Sometimes you’re not, frankly, always in the state of mind to look for the right things” like a medallion, a license, or the proper city-issued identification on a taxi, Kafka said. “If you’re using Hailo, the only thing you’re responsible for is pressing the right button.”

But LaGasse and Fogarty expressed concerns about the smartphone apps.

“Everyone likes technology, including the taxi industry, as long as it’s used responsibly,” LaGasse said. “But we’re concerned about these rogue apps.”

Correction: An earlier version of this story mischaracterized Uber Boston’s drivers. The company uses both licensed taxis as well as Town Cars and people using personal cars who pass criminal background checks.